1. Field of the Invention
The present invention relates to navigation devices and traffic line display methods, and is suitably employed in a navigation device having a function of displaying traffic lines representing traffic congestion levels along roads in a map image.
2. Description of the Related Art
In general, a navigation device detects a current position of a vehicle using a self-contained navigation sensor and a GPS (Global Positioning System) receiver, and displays data of a map of a region around the current position read from a recording medium in a screen. A vehicle position mark representing the position of the vehicle is displayed in the screen so as to overlap on a certain position of the screen. Accordingly, a current position of the vehicle can be recognized on the view.
Most navigation devices in recent years are capable of receiving road traffic information transmitted from Vehicle Information and Communication System Center (VICS center: VICS is a registered trademark) and displaying traffic conditions which change from moment to moment in a navigation screen. The road traffic information can be obtained from light beacons mainly installed in general roads, radio wave beacons mainly installed in expressways, and FM multiple broadcasting utilizing FM broadcasting radio waves.
Such a navigation device utilizes road traffic information and displays traffic congestion conditions (traffic congestion levels of various sections) of roads in a screen so as to notify a driver of the traffic congestion conditions. The road traffic information includes three types of information including first level information (text display), a second level information (simple graphic display), a third level information (map display).
The first-level road traffic information corresponds to character information in which a driver is notified of traffic information and a period of time required for arriving at a certain point by text. The second-level road traffic information corresponds to simple graphic information (image data using a bitmap, for example) in which a driver is notified of traffic information and a period of time required for arriving at a certain point by a simple graphic. In the simple graphic information, congestion sections (red) and crowded sections (orange) are displayed in a default road image provided by the VICS center in advance.
The third-level road traffic information corresponds to map information in which traffic information is indicated by colored arrow marks on a map. In this map information, in accordance with traffic information (information on a congestion section, information on a crowded section, or information on a normal section) of links (vectors representing finely divided roads and lanes) transmitted from the VICS center, blinking red traffic lines representing congestion sections and blinking orange traffic lines representing crowded sections are displayed along roads corresponds to the links. Furthermore, blue traffic lines are displayed along roads in which vehicles are normally traveling in the map.
Note that a road traffic information display device capable of easily selecting desired road traffic information only by touching a region of a touch key assigned to a portion of road traffic information displayed in the screen by a finger even when a plurality of types of road traffic information such as traffic information, restriction information, and parking information are mixed has been proposed (refer to Japanese Unexamined Patent Application Publication No. 9-236439, for example). Furthermore, a device capable of displaying, even when a region in map information is displayed as large area display in a scale larger than a predetermined scale, traffic information can be displayed along a road which is omitted and is not displayed has been proposed (refer to Japanese Unexamined Patent Application Publication 2007-232427, for example).
In expressways, ramps (connection paths which connect a plurality of expressways with one another) may be crowded, especially in expressways that connect cities with large traffic volume. However, there arises a problem in that, in a known technique, if traffic lines are displayed along individual expressways around ramps where a plurality of expressways are concentrated, a traffic line may be hidden by another expressway other than an expressway corresponding to the traffic line, or the relationships between the traffic lines and the corresponding expressways are not clear since the plurality of traffic lines are displayed in a mixed manner. For example, in a portion where a plurality of expressways intersect with one another, since at least a portion of a traffic line of an expressway located in a lower layer is overlapped by a traffic line of another expressway located in a higher layer, for example, the two traffic lines are seen as a lump of mixed color having no meanings. Therefore, the relationships between the traffic lines and the expressways are not clear.
Furthermore, in addition to the expressways, in national roads and prefectural roads, the relationships between traffic lines and the roads may be complicated in portions where a plurality of roads are concentrated.